The present invention relates generally to a brake control system for use in a motor vehicle, more particularly to such a brake control system which optimally controls the braking pressure on turning of the motor vehicle.
Anti-skid control systems are known as systems for controlling the speed of each of the wheels of a motor vehicle to maximize the frictional force between a tire and a road surface by causing the slip ratio S [(vehicle speed-wheel rotational speed)/vehicle speed] to be controlled at approximately 20%. This prevents the vehicle from suffering a decrease in safety due to locking of the wheels when braking. An example of this is a loss of control of the vehicle due to the locking of the front wheels and hip-swinging (skid) due to the locking of the rear wheels. (One example of such a system is disclosed in Japanese Provisional Publication No. 57-11149.)
In anti-skid control systems of this type, in order to maximize the frictional force between the tire and road surface and allow the vehicle to stop quickly without causing locking of the wheels, the rotational speed of each of the wheels is compared with a reference speed V derived on the basis of a vehicle speed Vs, i.e., given by V=Vs (1-S/100) where S represents the slip ratio (about 20%) and the braking hydraulic pressure is up-and-down controlled so that the rotational speed of the wheel becomes equal to the reference speed. That is, when the rotational speed of the wheel is below the reference speed, the control system causes adjustments to decrease the braking pressure applied to the wheel. When rotation speed exceeds the reference speed the control system causes an increase in the braking pressure. This braking pressure control is repeatedly performed so that the rotational speed of the wheel becomes closer to the reference speed.
Furthermore, it is known that the braking pressure can be controlled on the basis of the inclination degree of the vehicle body without the generation of excessive braking force (Japanese Provisional Publication No. 49-8690).
Such prior art systems suffer no problem as long as the motor vehicle runs straightforward. However, when the motor vehicle is turning, appropriate control for each of the wheels is not performed since slip conditions of the left and right wheels are determined in accordance with a level relating to vehicle speed and inclination degree of the vehicle body. Therefore, when braking is performed during turning of the vehicle, the prior art systems have a disadvantage in that the brake control start timings for the left and right wheels are different from each other. This results in different steering sensitivity when turning than is a normal driving condition.